The viability of New Zealand’s infrastructure
By Professor John Tookey, Head of Department Built Environment Engineering, Auckland University of Technology.
The structural failure of modern buildings, built with modern structural systems and to modern earthquake design codes is horrifying to many. Given that engineers routinely over design structural solutions to a safety factor of between 1.5 and 2.5 times the ‘worst case’ loading. Thus, it instantaneously puts into question the viability of so much of our infrastructure.
Not unreasonably the general public and engineers want answers and want them quickly. When the dust settles in Wellington and surrounds, investigations will be thorough and exhaustive.
In common with any engineering disaster – or near disaster – of this type, the suspects are always the same. It will come down to either design failure, materials failure, construction process failure or flawed engineering assumptions – very often a combination of these features. This last feature – flawed assumptions - is instructive.
In truth, engineering assumptions are our Achilles Heel as engineers. Lines have to be drawn somewhere. We can’t design for everything before it becomes prohibitively expensive. The designers of the Titanic assumed flooding of four compartments was worst case – the iceberg breached five.
The case study of the Citigroup Centre in New York in the late 1970s is similarly disturbing – and worth reading. Engineers designed the building to cope with wind acting perpendicular to the structure as per building design regulations. But details of the design meant that the building was vulnerable to hurricane force winds acting at oblique angles. Quite literally the building could have been blown over with mass casualties. Rectification took several months of work and was kept from the public for nearly 20 years.
I could go on and on. Ahead of the findings of any future inquiries on this catastrophic event, huge lessons will be learned. Professional engineers will stop and rigorously evaluate what went wrong. Design codes will be reassessed and redefined. Existing buildings will be assessed in the light of the findings. Rectification works will be undertaken on affected buildings. Current engineers will be retrospectively upskilled to reflect the findings. Engineering education will be amended to reflect the new knowledge – making engineers of the future that much more knowledgeable.
The damage is still fresh and the wounds are still raw. Perversely the outcomes of these events in the future is a better, safer, more sustainable built environment.
Concept for Modern Building Design for Severe Earthquake
By Dr Charles Clifton, Associate Professor of Civil Engineering and Structures Group Leader, University of Auckland.
We design modern buildings for defined conditions of performance in three levels of earthquake. These levels are called Limit States:
- The Serviceabililty Limit State (SLS) earthquake is expected to occur with around 90 per cent probability in the 50-year design life of a typical building.
- The Ultimate Limit State (ULS) earthquake is expected to have 10 per cent probability of occurrence 50-year design life of a typical building.
- The Maximum Considered Event (MCE) earthquake is expected to have approximately a two percent probability of occurrence in 50-year design life of a typical building.
The earthquake actions generated by these limit states, for buildings up to around 10 to 15 storeys high, are very large compared with those of other lateral loading conditions such as wind. The SLS earthquake actions, for example, will be higher than the ULS wind (which has a 400 year return period); the ULS earthquake actions will be some five times higher than the SLS earthquake actions and the MCE earthquake actions some eight times higher than the SLS earthquake actions.
The ULS and MCE earthquakes are low probability events. To design a typical building to remain undamaged in these events is a major cost and so buildings are typically designed to undergo controlled damage in these levels of earthquake, protecting the occupants at the expense of controlled structural damage. Through this tradeoff of strength for damage, the ULS design actions for the building structural system can be lowered to the SLS level, but at the expense of structural damage being generated when the earthquake exceeds the SLS level.
A good analogy for this is design of a car:
- The SLS earthquake is the most likely minor accident in a supermarket car park or the local high street. There may be some panel damage but the car remains fully functional.
- The ULS earthquake is the less likely accident on a major suburban road. The car will be damaged and will need repair or at worst replacement but the occupants will be safe and expectedly uninjured.
- The MCE earthquake is a head-on crash on the open road. Hopefully the occupants will survive OK; the building will be damaged beyond repair.
We have considerable experience now in New Zealand on how well our modern buildings perform in the different levels of earthquake intensity. Generally, the performance is demonstrably better than these limits. For example, the Christchurch 2010/2011 series of six damaging earthquakes (i.e. above SLS level) in the CBD comprised a MCE event in intensity and duration, but which took place over some 18 months instead of in one initial very large event with a series of typical aftershocks. Only the CTV building collapsed; some modern medium to high rise buildings survived with no structural repair needed. Other recent earthquakes have shown that for most buildings the damage threshold is some two to three times higher than the models would indicate.
How Severe Was the Kaikoura Earthquake in Wellington?
The best measure of damage potential is the intensity, expressed by the Peak Ground Acceleration (PGA). Duration also matters for severe earthquakes but is less critical.
The ULS design level PGA for a typical building in Wellington is 0.40g, where g is the acceleration due to gravity. (Aircraft turbulence on a very rough flight is around 0.2g to 0.25g.) The serviceability level is around 0.1g. Damage threshold for a modern building designed for maximum controlled damage is around 0.15g.
“The Kaikoura earthquake generated PGA values in Wellington of between 0.2g and 0.3g. So the widespread level of structural damage in modern buildings designed for controlled damage in a severe earthquake is to be expected.”
Why are some buildings more affected than others?
This depends on the size and shape of the building and the type of ground it is on. Soft reclaimed ground moves more violently in an earthquake than solid ground, hence is more damaging to buildings. If the ground shakes it starts to develop a resonant shaking frequency, such as you can get shaking a bowl of jelly, and this matches the vibration response of the building, the damage will be higher.
Also earthquakes expose weaknesses in design or construction of a building, such as inadequate tying together of structural components. All this leads to complex and considerable differences in building response, even in areas of close proximity. This wide variation is especially the case in an earthquake of the intensity somewhere between the SLS and ULS limit states, as some buildings will remain elastic and others will undergo damage. When the earthquake gets very severe – e.g. as in the 22 February, 2011 Christchurch earthquake – everything is damaged to some extent.
Response spectra for soils
It is clear that for buildings on class D soils, which is all the Lambton Quay and downtown area and much of the Taranaki St and Courtenay place area, if the building period (i.e. the time it would take if you pulled the top of a building over sideways, let it go and measured how long it took to sway to the other side and come back again) is between about 0.9 to 1.5 seconds, this has been closer to the ULS event in intensity than to the SLS event. Modern, medium rise, flexible buildings are in that range so this explains why damage is concentrated into those buildings on soft soils.